1. Field of the Invention
This invention relates to an ultrasonic transmit-receive apparatus for detecting underwater objects, for instance, by transmitting and receiving ultrasonic waves as well as to a scanning sonar employing the ultrasonic transmit-receive apparatus.
2. Description of the Prior Art
Conventionally, scanning sonars are used for detecting underwater targets. Typically, a scanning sonar is provided with a generally cylinder-shaped transducer like the one shown in FIG. 1 for detecting underwater targets around a vessel on which the scanning sonar is installed. The scanning sonar produces an ultrasonic transmitting beam directed in all directions around the transducer by causing its transducer elements to oscillate. Referring to FIG. 1, the individual transducer elements of the transducer are driven by signals which are precisely phase-controlled to introduce appropriate time delays in driving the transducer elements of one ringlike horizontal array (or row) to another, so that the ultrasonic transmitting beam forms a full-circle umbrellalike pattern of a narrow vertical beamwidth directed obliquely downward by a specific tilt angle. In each successive receive cycle, the scanning sonar produces a pencil-shaped receiving beam directed obliquely downward in a particular azimuth (horizontal direction) by driving a specific number of vertical arrays (hereinafter referred to as columns or staves) of transducer elements oriented toward particular directions. The combination of transducer element staves is sequentially switched along the circumferential direction of the transducer so that the receiving beam is successively rotated from one direction to another around the transducer.
The transducer of the aforementioned scanning sonar is constructed by xe2x80x9ccoarselyxe2x80x9d arranging a plurality of transducer elements. Due to this structure, the conventional transducer produces grating lobes and receives reflections from other directions than an intended sounding direction, eventually producing false images on-screen as a consequence.
FIG. 6 is a diagram showing an example of grating lobe formation. The transducer creates a main lobe ML having sharp directivity in the direction of an alternate long and short dashed line. The transducer also creates grating lobes GL directed generally perpendicular to the main lobe ML as well as multiple side lobes SL on both left and right sides of the main lobe ML.
Upon scanning underwater situations by successively rotating the receiving beam having such directional properties, the scanning sonar displays a picture as shown in FIG. 7, for example, in which designated by R is a real image of a target and designated by I are false images of the target. The false images I like this are created if any of the grating lobes GL or the side lobes SL is oriented toward the actual target (existing in the direction of R) and receives echoes from the target when the main lobe ML is emitted in the direction of either of the false images I.
One approach to attenuating the grating lobes GL is to reduce intervals between adjacent transducer elements by increasing the total number of transducer elements. Although this approach is effective to some extent, an increase in the number of transducer elements leads to an increase in the number of such circuits as drive circuits for driving the individual transducer elements, control circuits for controlling the drive circuits, receive circuits and signal processing circuits for processing received signals, eventually causing an increase in the scale of hardware and a considerable cost increase.
Accordingly, it is an object of the invention to provide an ultrasonic transmit-receive apparatus and a scanning sonar employing the ultrasonic transmit-receive apparatus which can solve the aforementioned false image problem and offer a higher angular resolution without causing an increase in the scale of hardware or in manufacturing cost even if a transducer is so structured to produce relatively large grating lobes and side lobes.
According to the invention, an ultrasonic transmit-receive apparatus comprises a transducer having multiple transducer elements arranged on a surface of the transducer at least along one direction in a horizontal plane, a transmit controller for driving the multiple transducer elements, causing them to emit ultrasonic waves, and a receive controller for synthesizing echo signals received by the individual transducer elements to produce a synthesized received echo signal upon receiving ultrasonic waves reflected by a target. The transmit controller drives the multiple transducer elements of the transducer in such a manner that the transducer elements emit the ultrasonic waves in directionally varying frequency bands, and the receive controller obtains received signals of specified angular directions by selecting the frequency from one angular direction to another.
Since the transmit controller causes the multiple transducer elements to emit the ultrasonic waves at directionally varying frequencies and the receive controller causes the multiple transducer elements to receive selectively echo signals of directionally varying frequencies from one angular direction to another, the echo signal received from each direction is substantially protected against interference with the echo signals received from other directions. As a result, it is possible to prevent false images from occurring due to grating lobes and side lobes and to offer a higher angular resolution.
In one feature of the invention, the transmit controller drives the individual transducer elements in such a manner that the ultrasonic waves emitted by the multiple transducer elements are equivalent to ultrasonic waves emitted from an imaginary moving sound source which moves within a circle enclosed by the multiple transducer elements arranged along the aforesaid one direction in the horizontal plane.
According to this feature of the invention, the frequency of a transmitting beam smoothly varies from one angular direction to another. This transmission method, referred to as the Doppler transmission method in the following detailed description, makes it possible to finely adjust the center frequency of the passband of a bandpass filter through which the received signals are passed to each angular direction and to easily increase the angular resolution.
In another feature of the invention, the transmit controller drives transducer elements necessary for forming a specific transmitting aperture which are chosen from the multiple transducer elements arranged along the aforesaid one direction in the horizontal plane by controlling the amounts of time delays of the ultrasonic waves to be emitted from the chosen transducer elements, wherein the transmitting aperture is successively moved from one direction to another such that a transmitting beam is steered from one angular direction to another all around the transducer.
In this transmission method, referred to as the rotational directional transmission (RDT) method in the following detailed description, a narrow pencillike transmitting beam is successively formed in one angular direction to another. Therefore, it is possible to increase the amount of energy fed into the single transmitting beam, extend detection range and scan across a wider area.
In another feature of the invention, the transmit controller successively varies transmitting frequency of the transmitting beam from one angular direction to another.
In another feature of the invention, the transmit controller successively selects adjacent transducer elements arranged within a specific sector area (aperture) from the multiple transducer elements arranged along the aforesaid one direction in the horizontal plane, wherein the transmit controller causes the selected transducer elements to emit frequency-modulated ultrasonic signals while shifting the direction of the sector area along the aforesaid direction in the horizontal plane.
This transmission method, referred to as the rotational aperture frequency modulation (RA-FM) transmission method in the following detailed description, makes it possible to allocate desired frequencies to the individual directions with an increased degree of freedom and more easily avoid directional spurious emissions (false images) which could occur in the aforementioned Doppler transmission method in which the transmitting beam of the same frequency is formed in two different directions.
In another feature of the invention, the transmit controller feeds such drive signals into the individual transducer elements that phases of the drive signals generally match at the middle of envelopes of drive signal waveforms in all angular directions, and the transmit controller causes the multiple transducer elements to emit ultrasonic signals of which transmitting frequency varies from one angular direction to another.
This transmission method, referred to as the directionally varying frequency continuous wave (CW) transmission method in the following detailed description, also makes it possible to allocate desired frequencies to the individual directions with an increased degree of freedom and more easily avoid directional spurious emissions (false images) which could occur in the aforementioned Doppler transmission method in which the transmitting beam of the same frequency is formed in two different directions.
In another feature of the invention, the transmit controller includes a pulse expander for expanding drive signal pulses fed into the individual transducer elements by convoluting burst waves with an FM signal, and the receive controller includes a pulse compressor for compressing the received signals of each angular direction before or after the frequency is selected for each angular direction by convoluting the received signals with an FM signal of which time axis is reversed.
This transmission method, referred to as the pulse expansion method in the following detailed description, makes it possible to prolong the duration of each drive signal pulse, increase the amount of energy fed into the transducer elements per transmit cycle and consequently extend the detection range.
In another feature of the invention, the ultrasonic transmit-receive apparatus further comprises a side-lobe eliminator for eliminating side lobes occurring on the time axis by the aforementioned pulse expansion operation.
Since side lobe levels of the drive signal spectrum decrease, effects of suppressing adverse influence of grating lobes and side lobes and of improving angular resolution are further increased.
In still another feature of the invention, the transmit controller precalculates the waveform of ultrasonic signals to be emitted in each of sector areas into which the horizontal plane is divided, calculates an angular direction and the amount of time delay of the waveform of the ultrasonic signals according to motion of the transducer for forming a transmitting beam directed to a desired direction, and defines drive signals to be fed into the individual transducer elements.
This makes it possible to generate the drive signals to be fed into the individual transducer elements with little computational complexity and easily stabilize the transmitting beam according to the motion of the transducer.
In yet another feature of the invention, the transducer has a flat, cylindrical or spherical surface on which the multiple transducer elements are arranged.
According to this feature of the invention, it is not necessary to use transducer elements of special design or employ special arrangement the transducer elements on the transducer. Therefore, control operation for driving the transducer elements is simple, resulting in a reduction in overall manufacturing cost.
According to the invention, a scanning sonar comprises the aforementioned ultrasonic transmit-receive apparatus, and means for controlling the transmit controller and the receive controller of the ultrasonic transmit-receive apparatus to successively scan through specified angular directions, producing data on a detected echo image from the received echo signals and displaying the detected echo image.
These and other objects, features and advantages of the invention will become more apparent upon reading the following detailed description in conjunction with the accompanying drawings.